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Here's an odd one. Yesterday as I pulled out of my garage one of the wing mirrors fell off. Not the whole assembly, just the mirror glass. It was undamaged when I picked it up, so I assume that I can just glue it back, But my question is "what glue to use"? I asked at the local dealer, but they were baffled "...it comes a complete unit, we don't glue anything".

Given that the mirrors are permanently heated when the car is running, the glue has to be able to withstand I guess 80degF to 90degF or thereabouts. Any suggestions? Will good old superglue do the trick?

As an aside, I've always wondered why the mirrors are permanently heated when the car is running - any ideas?

... I've always wondered why the mirrors are permanently heated when the car is running - any ideas?...

Mirror defrosting draws too little current and it's expensive to add a switch.
My 2008 Forester LLBean has one switch for heated wiper perches in the windshield, and heated mirrors. The heating elements in the windshield are huge to melt solid ice, and draw a big current when on. They are worth switching off as they are not needed 99.99% of the time.
But heated mirrors just do light duty defrosting, do not do de-icing, don't draw much current, and are not worth switching in the Z3.

I bought the Burco inserts from eBay and replaced leaking dimming mirrors almost a year ago. They come with these "redi stick patches" (very thin double sided mirror tape, good for heated mirrors too). Give them a call and see if they'd sell you just the adhesive patches. 1-888-253-2593

Another option would be to silicone the mirror onto the plastic backing that the mirror originally sat on.
My advise, whatever you do, take off that plastic mirror backing and remove any and all adhesive material from it before you try to re-glue. When I did mine I actually used a putty knife to remove mirror from plastick back and then sanded the mirror tape off the plastic back.

Mirror heating is controlled by the door modules. The heating function is active as of terminal 15 (ignition lock position 2). The activation time is dependent on the outside temperature.
The general module receives the outside temperature from the IKE via the K-bus and determines a cycle time for heating operation. This time is transferred to the peripheral modules where it is carried out. Heating is no longer active at an outside temperature greater than 25 o C and it is switched on permanently at a temperature lower than -10 o C.

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.How does something immoral, when done privately, become moral when it is done collectively?
Furthermore, does legality establish morality?
Slavery was legal; apartheid was legal; Stalinist, Nazi, and Maoist purges were legal. Clearly, the fact of legality does not justify these crimes. Legality, alone, cannot be the talisman of morel people. - Walter E. Williams

I came across this website and the advice offered made sense. So I bought a tube of silicone RTV adhesive/sealant. I cleaned the heating element and the back of the mirror with lacquer thinners and dotted the heater pad as recommended. When you set the mirror on the pad, try to avoid transferring adhesive to the mirror surface. but if you do, Rainex will take it off tout suite!